Gamma-ray bursts (GRBs) are the most energetic phenomena in the universe. To elucidate the radiation mechanism of GRBs, we are developing a Compton-scattering-type polarimeter using a plastic scintillator and CsI(Tl) scintillator attached to a silicon photomultiplier (SiPM). We constructed a small test model and investigated the basic performance. Moreover, irradiating the SiPM with protons, the radiation hardness has been also investigated. From these experiments, we have recognized that the Compton-scattering-type polarimeter will obtain the sensitivity to the energy above approximately 30 keV if the radiation damage to the SiPM is not serious. On the other hand, the dark current of the SiPM becomes several thousand times higher for a few krad of irradiation and can be accumulated in the space environment in a few years. This means that the polarimeter will work well in balloon-borne experiments for a few weeks.